Blood oxygenator systems are used in open heart surgery and for providing emergency cardiopulmonary assistance. In both instances, the oxygenator takes over, either partially or completely, the function of removing carbon dioxide from the blood, and replacing it with oxygen, as is normally done by the patient's lungs.
A typical use of a blood oxygenator is shown with respect to FIG. 12, in which venous blood is removed from a patient and placed in a venous reservoir. A blood pump pumps the blood through an oxygenator which replenishes the oxygen in the blood. A heat exchanger adjusts the temperature of the blood to induce hypothermia or to maintain normothermia during surgery. The oxygenated blood then passes through an arterial filter to remove any bubbles, whereupon it is returned to the patient.
Within the oxygenator itself, the venous blood which is depleted in oxygen and enriched in carbon dioxide, is placed in contact with microporous membranes. The membranes have an enriched oxygen gas on one side, and the depleted oxygen blood on the other side of the membrane. Oxygen passes from the gas, through the membrane, into the blood. Concurrently, carbon dioxide passes from the blood, through the membrane, into the oxygen gas. The oxygenated blood is returned to the patient.
There are two types of membrane blood oxygenators currently available. The first type is referred to as a flat plate membrane oxygenator, and employs one or more thin, flat sheets of microporous membrane. Oxygen is placed on one side of the membrane, and oxygen-depleted blood is placed on the other side of the membrane, with the gas transfer taking place across the membrane.
The other type of membrane oxygenator is the hollow fiber oxygenator. This type of oxygenator uses hundreds or thousands of microporous or semi-permeable hollow fibers to achieve the gas transfer. The hollow fibers are sealed in the end walls of a housing such that a gas can be passed through the length of the hollow fibers. Blood is passed around the outside of the fibers with the gas transfer occurring across the walls of the plurality of fibers. In some devices, the blood flows through the hollow fibers with the oxygen gas flowing around the outside of the fibers to achieve the gas transfer.
The hollow fiber blood oxygenators typically have the fibers packed in a cylindrical shaped bundle, with the bundle length and the diameter varying depending upon the amount of gas transfer area desired. The housing into which these cylindrical bundles are placed is sized to correspond to the bundle diameter and length in order to ensure that all of the blood contacts the fibers, and contacts as many fibers as possible. Moreover, a close fit between the fiber bundles and the housing is desirable since it reduces the "priming volume" of fluid needed to fill the housing and fiber bundle and prevents blood from shunting around or bypassing the fiber bundle.
It is difficult, however, to maintain an accurate diameter on the fiber bundles because of the small size and flexibility of the individual fibers. For example, the hollow fibers can have an inside diameter of about 400 microns with a wall thickness of about 25 microns. These fibers are formed into a fiber bundle by winding them onto a core, and during the winding, the fibers may be pulled and stretched resulting in physical dimensional variability. The small size of these fibers thus makes it difficult to maintain accurate dimensions on the diameter of the wound fiber bundles which, in turn, increases manufacturing and assembly costs of blood oxygenators having close tolerances on the fit between the fiber bundle and the housing. There is thus a need to provide a more simple and efficient means for insuring a close fit between the housing and the fiber bundle in order to provide for a low priming volume and prevent blood from bypassing the fibers in the fiber bundle.
A heat exchanger is usually used in conjunction with a blood oxygenator in order to control the temperature of the blood returned to the patient. The heat exchange is typically achieved by passing the blood over a heated surface. There is a need, however, for an efficient, compact heat exchanger having a low priming volume and low flow resistance.